This invention relates generally to traction motor control systems, and more particularly to traction motor dynamic braking systems in locomotives.
In a conventional diesel-electric locomotive, electric traction motors provide the motive force to move the train. Typically, a diesel engine drives an alternator, which supplies current to drive traction motors, which, in turn, propel the locomotive forward or backward. When propelled as such, a locomotive is said to be motoring.
The traction motors, however, perform an additional function. Once the locomotive is in motion, traction motors may be configured to generate electricity instead of consuming it. As generators, the traction motors convert the locomotive's kinetic energy into electrical energy, thereby slowing the locomotive. Using the traction motors to reduce speed is called dynamic braking. Because there is no suitable storage medium for the generated electrical energy, an electrically resistive grid, known as a braking grid or load box, is used to convert the electrical energy into heat energy, which is vented to the atmosphere.
In a typical diesel locomotive, a pair of traction motors are connected in parallel, and a resistive grid is connected in series between them. When the locomotive is motoring, the voltage drop across each traction motor is similar in magnitude and polarity, and as such, there is an insignificant voltage drop across the resistive grid. While using dynamic braking, however, the polarity of one of the traction motors is reversed, creating a substantial voltage drop across the resistive grid. Thus, in typical operation, the resistive grid is in constant electrical contact with the traction motors, yet dissipates energy only when the locomotive employs the dynamic braking technique.
A problem arises with conventional dynamic braking systems, however, when a resistive grid develops a short circuit to ground or to another element of the grid. Because the resistive grids are permanently coupled to the traction motors, a short circuit can interrupt the current flow to the traction motors. Consequently, the locomotive can be completely disabled by an element that isn't even involved in the actual propulsion of the locomotive. Short circuits may develop with water dripping into the resistive grid, or be caused by inadvertent damage during maintenance procedures. The likelihood of short circuits is enhanced by the fact that the grids are folded accordion style and packed tightly together to maximize heat transfer per area.
When a short develops, the massive amount of electrical energy generated to propel the locomotive is diverted to ground. Attempting to operate a locomotive with a grounded resistive grid practically ensures substantial damage to the locomotive's electrical generation and propulsion systems, as well as the resistive grid itself. Thus, a locomotive with a grounded resistive grid is completely disabled until the ground fault can be corrected.